The present disclosure is related to marking and printing methods and systems, and more specifically to methods and systems providing control of conditions local to the point of writing data to a reimageable surface in variable data lithographic system.
Offset lithography is a common method of printing today. (For the purposes hereof, the terms “printing” and “marking” are interchangeable.) In a typical lithographic process a printing plate, which may be a flat plate, the surface of a cylinder, or belt, etc., is formed to have “image regions” formed of hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. The image regions are regions corresponding to the areas on the final print (i.e., the target substrate) that are occupied by a printing or marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by said marking material. The hydrophilic regions accept and are readily wetted by a water-based fluid, commonly referred to as a dampening fluid or fountain fluid (typically consisting of water and a small amount of alcohol as well as other additives and/or surfactants to reduce surface tension). The hydrophobic regions repel dampening fluid and accept ink, whereas the dampening fluid formed over the hydrophilic regions forms a fluid “release layer” for rejecting ink. Therefore the hydrophilic regions of the printing plate correspond to unprinted areas, or “non-image areas”, of the final print.
The ink may be transferred directly to a substrate, such as paper, or may be applied to an intermediate surface, such as an offset (or blanket) cylinder in an offset printing system. The offset cylinder is covered with a conformable coating or sleeve with a surface that can conform to the texture of the substrate, which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the imaging plate. Also, the surface roughness of the offset blanket cylinder helps to deliver a more uniform layer of printing material to the substrate free of defects such as mottle. Sufficient pressure is used to transfer the image from the offset cylinder to the substrate. Pinching the substrate between the offset cylinder and an impression cylinder provides this pressure.
Typical lithographic and offset printing techniques utilize plates which are permanently patterned, and are therefore useful only when printing a large number of copies of the same image (long print runs), such as magazines, newspapers, and the like. However, they do not permit creating and printing a new pattern from one page to the next without removing and replacing the print cylinder and/or the imaging plate (i.e., the technique cannot accommodate true high speed variable data printing wherein the image changes from impression to impression, for example, as in the case of digital printing systems). Furthermore, the cost of the permanently patterned imaging plates or cylinders is amortized over the number of copies. The cost per printed copy is therefore higher for shorter print runs of the same image than for longer print runs of the same image, as opposed to prints from digital printing systems, where the per-page cost is typically independent of the number of copies that are printed.
Accordingly, a lithographic technique, referred to as variable data lithography, has been developed which uses a non-patterned reimageable surface coated with dampening fluid. Regions of the dampening fluid are removed by exposure to a focused heat source (e.g., using radiation such as a laser light source). A temporary pattern in the dampening fluid is thereby formed over the non-patterned reimageable surface. Ink applied thereover is retained in regions corresponding to the removal of the dampening fluid. The inked surface is then brought into contact with a substrate (such as paper), and the ink pattern transfers to the substrate. The dampening fluid may then be removed, a new, uniform layer of dampening fluid applied to the reimageable surface, and the process repeated.
The patterning of dampening fluid on the reimageable surface in variable data lithography essentially involves using a heat source such as a laser to selectively boil off or ablate the dampening fluid in selected locations. This process can be energy intensive due to the large latent heat of vaporization of water. At the same time, high-speed printing necessitates the use of high-speed modulation of the heat source, which can be prohibitively expensive for high power lasers. Therefore, from both an energy and cost perspective, it is beneficial to reduce the total amount of laser energy that is needed to achieve pattern-wise vaporization of the dampening fluid.
However, one byproduct of the pattern-wise evaporation of dampening fluid is generation of a vapor cloud. This cloud can partially absorb energy from the laser being used to write onto the dampening fluid layer, thus reducing the laser power available for patterning the dampening fluid layer.
With reference to FIG. 1, a layer 32 of dampening fluid is shown over a portion of a reimageable surface 34 carried by imaging member 12. A key requirement of dampening fluid subsystem 14 is to deliver dampening fluid such that layer 32 is of a controlled and uniform thickness. In one embodiment layer 32 is in the range of 200 nanometers (nm) to 1.0 micrometer (μm), and very uniform without defects such as pinholes. The dampening fluid itself may be composed mainly of water, optionally with small amounts of isopropyl alcohol or ethanol added to reduce its natural surface tension as well as lower the evaporation energy necessary for subsequent laser patterning. In addition, a suitable surfactant may be added in a small percentage by weight, which promotes a high amount of wetting to the reimageable surface layer. In one embodiment, this surfactant consists of silicone glycol copolymer families such as trisiloxane copolyol or dimethicone copolyol compounds which readily promote even spreading and surface tensions below 22 dynes/cm at a small percentage addition by weight. Other fluorosurfactants are also possible surface tension reducers. Optionally the dampening fluid may contain a radiation sensitive dye to partially absorb laser energy in the process of patterning. In another embodiment, the dampening fluid may be non-aqueous, comprises for example of a fluid having a low heat of vaporization.
Typically, the thickness of the dampening fluid layer cannot be lower than about 200 nm (e.g., for an aqueous dampening fluid) to ensure reliable ink selectivity between hyodrophilic and hydrophobic regions over the reimageable surface, and the consequent contrast between the image and non-image zones. This is mainly because the selectivity for ink transfer is a result of the splitting of the sacrificial dampening fluid layer from the dampened regions of the reimageable surface, and a thinner dampening fluid layer may not split reliably.
This minimum required dampening fluid layer thickness of approximately 200 nm results in a minimum per-pixel energy requirement based on the heating requirements for boiling-off the dampening fluid (e.g., water), equal to the sensible heating (i.e., heat needed to raise the temperature of the water to its boiling point, typically from a room temperature of about 20° C. to approximately 100° C., which equals the specific heat capacity times the temperature rise of approximately 80° C.) and latent heating (i.e., heat or enthalpy of vaporization of water which is about 540 calories per gram at atmospheric conditions). Based on the above information, we can calculate the power requirements for laser based vaporization of a 200 nm thick layer of water for a print speed of 100 pages per minute and a resolution of 600 dpi (42 micron pixel size and pitch), as shown in Table 1, below.
TABLE 1Resolution600dpiThickness of dampening fluid0.2micronslayerPrint speed100ppmDot size (diameter)42.33micronsDampening fluid mass per pixel2.81E−13kgDampening fluid latent heat1.52E−07calrequired per pixelDampening fluid sensible heat2.11E−08calrequired per pixelTotal dampening fluid heat1.73E−07 cal (or 7.24E−07 J)required per pixelRequired minimum energy5.14E−02J/cm2densityNumber of pixels in a 8.5 × 11″33660000pixelspageTime per pixel1.78E−08secScanning laser power40.60Watt
The above are the theoretical minimum energy and power requirements for vaporization of the dampening fluid assuming that it is comprised only of water, and without accounting for heat loss into the reimageable surface or other regions of the system. It will be appreciated that a relatively high power laser source is required under ideal conditions. However, the cloud of dampening vapor resulting from prior boiling off of regions of the dampening fluid layer can absorb a significant amount of the laser source energy. Considering the presence of this cloud, higher laser power levels are needed to enable boiling-off of the regions of dampening fluid. Providing such a high power laser source may be prohibitive from a number of perspectives such as cost, energy consumption, and so on.
Furthermore, the cloud of vaporized dampening fluid can re-condense onto the fluid layer, partially filling and altering the wall profiles of the pockets created by laser writing process. This is especially true for dampening fluids containing large solids, where preferential edge development can be seen due to vapor cloud diffusion.
Still further, variations in surrounding air humidity can negatively impact the removal rate of dampening fluid from the dampening fluid layer. For example, if a water based dampening solution is used, a higher concentration of water molecules in the surrounding air results in a higher likelihood of re-condensation on areas that are intended to be free of dampening fluid, and an increase in evaporation resulting in more absorptive material interposed between the laser source and the dampening fluid layer as well as variation in layer thickness.